The relationship between elevated blood lipids, particularly cholesterol (and especially low-density-lipoprotein cholesterol) and atherosclerosis has been known for many years. More recently, reduction of LDL cholesterol by means of surgery or drugs has been shown to reduce the risk of coronary heart disease. However, the reduction of cardiac events achieved by cholesterol lowering does not correlate well with the relatively small amount of physical regression in the amount of atherosclerotic plaque seen in the coronary arteries following treatment. In addition, relief of angina pectoris (ischemic chest pain) often occurs in a matter of weeks following cholesterol lowering; whereas, documentable changes in the inside diameters of coronary arteries may take years to occur, if they occur at all. The pain associated with angina pectoris is attributable primarily to lactic acid produced when heart muscle cell metabolism occurs in the absence of oxygen. Coronary artery narrowing can limit the amount of blood-transported oxygen that reaches the heart muscle tissue, but, the above observation suggests oxygenation of heart muscle tissue can be improved without increasing blood flow through the coronary vessels.
The way in which changes in blood lipids, such as cholesterol, might affect oxygen delivery to heart muscle tissue has remained unclear. There is abundant oxygen in blood. In fact, oxygenated (arterial) blood contains approximately as many molecules of oxygen per 1000 mL as are found in 200 mL of oxygen gas. Almost all (98-99%) of this oxygen is bound to hemoglobin molecules within the red blood cells; the remainder is physically dissolved in plasma and intracellular red blood cell fluid. For oxygen to reach tissues, such as cardiac muscle tissue, oxygen must be released from hemoglobin and then diffuse across the red blood cell membrane into the plasma and from there into tissues. The movement of oxygen across the red blood cell membrane occurs by passive diffusion and is governed by concentration gradients; there is no active membrane transport system for oxygen. Furthermore, the composition of a subject's red blood cell membrane changes with changes in the subject's lipid status. Therefore, the red blood cell membrane can be a significant barrier to release of oxygen into tissue such as cardiac muscle tissue.
What is needed is a method and apparatus to assess the significance of the red blood cell membrane as a hindrance to oxygen transfer from blood to tissues, such as cardiac muscle tissue. Such a method and apparatus would provide a new way to assess heart and circulatory disorders related to oxygen transport, such as angina pectoris; a new way to measure, and to assess the impact of, a patient's blood lipid levels; and a new way to monitor the effectiveness of lipid-lowering therapies.